1. Field of the Invention
The present invention relates to a speed control device for an electric motor.
2. Description of the Related Art
A method of automatically adjusting a control constant in a speed control unit constituting a speed control device for variably controlling the speed of an electric motor is disclosed in JP-A-1-218389, for example. According to this method, a disturbance signal is applied to the output of the speed control unit to vary the speed of the electric motor, the total of the moment of inertia of the electric motor and the moment of inertia of a driving target is estimated on the basis of the speed of the electric motor and the current of the electric motor at this time, and then the control constant of the speed control unit is automatically set on the basis of the estimation value.
According to the conventional method of estimating the inertia moments of the electric motor and the driving target, it is required to rotate the electric motor through the speed control device and further inject a disturbance signal into the speed control device. However, it may be impossible to rotate the electric motor under some operating conditions of the driving target, so that the disturbance signal cannot be injected into the speed control device. Therefore, there is a problem that the control constant cannot be set in the speed control unit constituting the speed control device.